The present invention pertains to a method relating to a mobile telephone system, comprising locating a mobile station (MS), which mobile telephone system includes base stations (BS1, MT) which are connected to a mobile switching center (MSC) and to which a desired number of frequencies have been allocated.
Known mobile telephony systems which use TDMA, that is to say Time Division Multiple Access, and MAHO, that is to say Mobile Assisted Handoff, have in each base station a control channel whose signal strength can be measured by the mobile stations and whose frequency permits identification within a given geographical area of the transmitting base station. Examples of such systems are the European GSM-system and the American system which is specified according to a standard designated EIA/TIA IS-54. The land system can order the mobile station to measure and report the signal strength on given frequencies, which makes it possible to identify those base stations which lie closest to the mobile station.
There has been proposed in U.S. patent application Ser. No. 07/680,508 a mobile telephone system which is intended for use by subscribers both indoors and outdoors. The system includes an external part-system and an internal part-system. Adaptive channel assignment is applied in the internal part-system, which provides indoor users access to all traffic channels in the system. The transmitter powers are small in comparison with the transmitter powers in the external part-system, which enables the same traffic channels to be used in several cells within a building. The control channel belonging to each cell indoors is not adaptive but is fixed and, in a manner similar to that for the outdoor cells, identifies the base station of the mobile station. This mobile system includes a large number of indoor base stations. The fixed control channel is a drawback, since it can easily be subjected to disturbances from the outdoor system and is then not changed adaptively. Furthermore, it is difficult for the land system to clearly identify an indoor base station for handoff on the basis of the frequency of the control channels when a subscriber reports measurements made on the control channels of peripheral base stations during a call. This difficulty can occur when the subscriber chooses to pass from a street in a given outdoor cell into one of several possible buildings. The indoor base stations in these buildings must then be clearly defined in the reported measurements from the mobile, which requires the control channel frequencies in all of these buildings to be different.